The present invention relates to a continuously operating centrifuge especially one suitable for a medium quality product and/or for so-called white sugar massecuite. Such centrifuges have conventionally a centrifugal basket which rotates about a vertical axis. The basket has a conical shape, the larger diameter end of which faces upwardly. The centrifugal basket is provided with a screen on the inside of the basket. The basket is secured to the upper free end of a drive shaft. A distribution cup is also secured to said upper end of the drive shaft to reach into the centrifugal basket. Distribution pins are located inside the distribution cup slightly spaced from the wall of the distribution cup. An acceleration cone or bell extends from the distribution cup to the bottom of the centrifugal basket. The acceleration cone faces downwardly with its larger diameter end. Further, a filling device through which massecuite is supplied into the distribution cup, is part of the centrifuge.
Numerous attempts have been made to develope an operational, continuously working centrifuge, capable of handling a product of average quality or so-called white sugar massecuite in an economic manner and simultaneously with sufficient quality. To achieve such a goal it is necessary to overcome two substantial difficulties. One problem resides in the fact that a product produced in a continuously operating centrifuge does not have the desired purity because the so-called washing or purging is insufficient in a continuously operating centrifuge to remove syrup remainders to the extent necessary for producing white sugar. The other problem resides in the fact that in a continuously operating sugar centrifuge, the sugar crystals are subjected to mechanical wear in an undesirable extent. A portion of the mechanical wear is caused by the sliding movement of the sugar crystals inside the centrifuge. However, a substantially serious damage to the sugar crystal results because the sugar crystals bounce against a fixed wall after passing a relatively short pass when they are ejected at the upper end of the centrifugal basket, especially since the ejection speed corresponds to the circumferential speed of the upper edge of the basket.
Heretofore, those skilled in the art have been of the opinion that it was merely necessary to keep the sugar in a sufficiently thick layer for an appropriate length of time in the centrifugal basket of a continuously operating centrifuge in order to perform the washing operation in such a manner that the quality of the product corresponds to that produced in batch type centrifuges.
Thus, U.S. Pat. No. 3,799,353 and German Patent Publications Nos. 21, 51, 476 as well as 21, 51, 475 disclose a continuously operating centrifuge of the so-called thick layer flow type constructed to produce white sugar from a respective white sugar massecuite.
In such a centrifuge the massecuite is supplied through a vertical filling pipe into a conical acceleration device opening downwardly. This acceleration device is constructed as an accumulation bell which may be lifted and lowered. This feature is intended to accelerate the supplied massecuite to the circumferential speed even prior to the time the massecuite reaches the centrifugal basket. The centrifugal basket comprises a screen and its slope varies in stages, the upper edge of the basket is provided with a so-called accumulation ring which defines together with the upper edge of the basket, a passage gap for the sugar. A dosing slide is arranged radially outside of the passage gap for the sugar. The slide has a downwardly slanting collar forming a ring. The ring collar is rigidly secured by means of bolts to the centrifugal basket and spring biassed to keep the ring collar in a starting position in which its downwardly directed portion blocks the flowing movement of the sugar through the sugar discharge gap. A magnet held in a fixed position is arranged to lift the dosing slide relative to the centrifugal basket when the latter is in a predetermined rotational position so that in this position the sugar discharge may take place. The stroke of the dosing slide is controllable by the adjustment of the energizing of the magnet.
The just described elements of the prior art are intended to produce on the screen of the centrifugal basket a very thick sugar layer by the respective accumulation resulting from the control of the dosing slide. Such a thick layer is supposed to be chargable with washing water in the same manner as is possible in a batch type centrifuge operating with a thick sugar layer. In addition, the just described prior art elements are intended to provide a relatively longer residence time of the sugar within the centrifuge.
Disregarding for the moment that thick layers of material may cause critical and even dangerous operational conditions especially where the massecuite may have an inhomogeneous structure due to locally different flow characteristics, simple considerations will show that it is not possible to produce white sugar of a sufficient quality by employing the thick layer principle alone. For example, the quantity of the liquid component of the massecuite becomes the larger, the thicker the layer of the material on the screen of the centrifugal basket. This means in turn, that a material layer of a certain thickness requires a corresponding length of time for the removal of the liquid component from the material. The washing water also requires in the same sense more time for penetrating a thick layer. The washing water must also be removed as so-called high green syrup. The flow path of the massecuite is much too short in the known thick layer flow centrifuge to provide for the required lengths of time. Thus, in order to produce white sugar of the corresponding or sufficient quality, it would be necessary that the dosing slide blocks the sugar exit until sugar of suitable quality results from the washing. For the same lengths of time, the control means would have to interrupt the supply of further massecuite. Only after sugar of the desired quality has been produced would it be possible to open the dosing slide, to discharge the sugar and to replace the layer of material in the centrifuge by an inflow of massecuite. However, as soon as the massecuite reaches the upper area adjacent to the edge of the centrifugal basket, the dosing slide would have to be closed instantaneously to make it possible to process the material in the centrifuge until the desired quality is obtained. This type of operation is thus only suitable as a theoretical possibility and it is not possible to realize an economic production sequence because the through-put per unit of time is much too small. Furthermore, the above mentioned publications do not disclose any teaching how the sugar quality could be monitored.
Further substantial drawbacks in the practical construction of the known centrifuge are seen in that the electromagnet is not capable to timely operate the dosing slides against the spring forces and against the prevailing mass forces. Moreover, it is quite likely that a non-symmetry in the loading of the centrifuge will result when the dosing slide is lifted against the force of the springs which tend to hold it down. This may result in an uneven rotation and possibly heavy vibrations of the centrifuge. Thus, the prior art also suggests to lift and lower the dosing slide by air pressure or by mechanical means. However, such solutions are technically unsound.
German Pat. No. 12, 72, 229 also addresses itself to realizing a continuously operating white sugar centrifuge. In this German Patent it is suggested to use an acceleration cone which is provided at its lower, wide end with only a small passage gap for the material to be centrifuged. This gap faces in the axial direction against a screen. By these means it is intended to achieve a sedimentation in the acceleration cone of the material to be centrifuged. By these means it is intended to cause the sugar crystals to accumulate on the wall of the acceleration cone while the liquid phase is supposed to be separated radially inwardly, that is, on the crystal layer so that it may flow off through the screen. In this manner, it is supposed to be possible to operate the centrifugal basket with a lower rpm, whereby to reduce break-up or damage to the sugar crystals. However, this prior art teaching cannot be realized in practice, because the desired sedimentation does not take place to the extent necessary for the removal of a substantial proportion of the liquid quantity.
Even if one assumes the desired degree of sedimentation may be accomplished, this would still be of substantial disadvantage to the quality of the sugar because the sugar crystals would be subjected to a dry sliding movement on the surface of the acceleration cone. In addition, the crystals would be subjected to an increased gravity action and slide relative to each other. As a result, crystal destruction and crystal rub-off could not be avoided and sugar of the desired quality could not be produced. Besides, if the known centrifuge is supposed to operate with rpms lower than normal, the intended sedimentation becomes still more unlikely. In the known centrifuge, the massecuite is supplied off center into the acceleration cone, whereby radially extending wings accelerate the massecuite. These wings beat the massecuite and cause damage to a substantial proportion of the crystals due to the instantaneous acceleration. Thus, a uniform, careful distribution of the massecuite over the surface of the cone and a respectively gentle acceleration are impossible in the known apparatus. Hence, known centrifuges are not suitable for centrifuging of white sugar massecuite, nor can they be used for this type of work with high efficiency and large through-put capacities due to said unsatisfactory accelerations.